Assembling Bi2MoO6/Ru/g-C3N4 for Highly Effective Oxygen Generation from Water Splitting under Visible-Light Irradiation

Article abstract of DOI:10.1021/acs.inorgchem.9b00524

Water oxidation is a kinetically challenging reaction for photocatalytic overall water splitting. Producing one molecule of O₂ will consume four electrons, so it is an extremely difficult obstacle for researchers. Here, a Bi₂MoO₆/Ru/g-C₃N₄ composite was obtained by a gentle hydrothermal method, which could oxidize water into O₂ highly efficiently. The optimal O₂ production reached 328.34 μmol·g^-1·h^-1 under visible light irradiation. Moreover, the catalyst presented excellent stability, as shown by a still sustentative 91.4% photocatalytic activity and invariant textural structure after seven recycling tests. The ternary material had the smallest resistance, which indicated that it has a good photoelectron conductive tunnel, and a rapid transfer route is proposed through Bi₂MoO₆ → Ru → g-C₃N₄ → NaIO₃ (electron acceptor). The massive holes (h⁺) with high oxidative potential are surely enriched due to the quick electron migration, being fit for a large promotion of the multiple-electron water oxidative proceedings. Therefore, the metallic Ru provided a powerful bridge for efficient transfer of the interface electrons which could be beneficial to spatial separation of photoexcited carriers without the loss of the high redox capacity. Finally, it is proposed that the Ru-assisted electron transport and constituent synergy in Bi₂MoO₆/Ru/g-C₃N₄ composite play crucial roles to its enhanced light utilization, efficient photoelectric conversion property, and high-producing oxygen capability.

Full text of DOI:10.1021/acs.inorgchem.9b00524

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Assembling Bi2MoO6/Ru/g-C3N4 for Highly Effective Oxygen
Generation from Water Splitting under Visible-Light Irradiation
Yuhang Wu, Meiting Song, Zhanli Chai, and Xiaojing Wang*
Inner Mongolia Key Laboratory of Chemistry and Physics of Rare Earth Materials, College of Chemistry and Chemical Engineering,
Inner Mongolia University, Hohhot, Inner Mongolia 010021, P. R. China
S
* Supporting Information
ABSTRACT: Water oxidation is a kinetically challenging reaction for
photocatalytic overall water splitting. Producing one molecule of O2
will consume four electrons, so it is an extremely difficult obstacle for
researchers. Here, a Bi2MoO6/Ru/g-C3N4 composite was obtained by
a gentle hydrothermal method, which could oxidize water into O2
highly efficiently. The optimal O2 production reached 328.34 μmol·
g−1·h−1 under visible light irradiation. Moreover, the catalyst
presented excellent stability, as shown by a still sustentative 91.4%
photocatalytic activity and invariant textural structure after seven
recycling tests. The ternary material had the smallest resistance, which
indicated that it has a good photoelectron conductive tunnel, and a
rapid transfer route is proposed through Bi2MoO6 → Ru → g-C3N4
→ NaIO3 (electron acceptor). The massive holes (h+) with high
oxidative potential are surely enriched due to the quick electron
migration, being fit for a large promotion of the multiple-electron water oxidative proceedings. Therefore, the metallic Ru
provided a powerful bridge for efficient transfer of the interface electrons which could be beneficial to spatial separation of
photoexcited carriers without the loss of the high redox capacity. Finally, it is proposed that the Ru-assisted electron transport
and constituent synergy in Bi2MoO6/Ru/g-C3N4 composite play crucial roles to its enhanced light utilization, efficient
photoelectric conversion property, and high-producing oxygen capability.
of solar water splitting in terms of their cheap, environmentally
friendly, and relatively nontoxic properties as well as their
simplicity and stability. Various semiconductors such as
SrTiO3:Rh, Bi4TaO8Cl, TiO2:Rh,Sb, TiO2:Cr,Sb, and Ir/
CoOx/Ta3N5 show noticeable WOCs activity.18−22
1. INTRODUCTION
Solar energy induced water splitting to generate H2 and O2 is a
highly promising method for the issues of the energy crisis and
environmental pollution.1−3 There are two ways to split water:
one is the overall water splitting by only one kind of catalyst
under the sun’s rays that is very difficult to reach, because the
catalyst is required to have a proper band gap, strong redox
capability, and high light utilization. Another relatively easy
way is to combine two semireactions, namely, the oxidation to
O2 and the reduction to H2 of water.4,5 H2 production by water
splitting has been widely researched in recent years.6−8
However, O2 evolution is much more difficult kinetically and
energetically, which will consume four electrons for the output
of an oxygen molecule.9 Therefore, to develop outstanding
water oxidation catalysts (WOCs) is of great importance for
solar energy induced water splitting.10−12
For the past few decades, various homogeneous transition-
metal multi- or mononuclear complexes with organic ligands
have proven to be effective WOCs such as Ru, Ir, Mn, and Co
complexes.13−17 However, the drawbacks of these precious and
transition metal complexes are the unstablility, high prices, and
harsh reactive conditions for any practical applications.
Therefore, in order to be economically viable, developing
powdered semiconductor WOCs under mild reaction con-
ditions will be expected, in order to give practical applications
As WOCs, the materials should have overpotentials larger
than H2O/O2 (1.23 eV vs NHE). Bi2MoO6 is a layered
Aurivillius related oxide with the corner-sharing cell of MoO6
octahedra.23 It has reported that the effective connection of
MoO6 octahedra could provide a tunnel for carriers trans-
ferring to promote the catalytic oxidative ability.24 Moreover, it
has a suitable band gap and very positive valence band
potential.25,26 Thus, Bi2MoO6 is considered to be a high
performance visible-light-driven photocatalyst,27−29 and its
photocatalytic activity has been established to be well
improved by hybridizing other semiconductors with a suitable
band potential. Many Bi2MoO6-based composites have been
27
reported, such as graphene/Bi2MoO6 and carbon quantum
dots/Bi2MoO6.28 Even so, it is still challenging to apply these
composite materials for the water oxidation, since (1) oxygen
production needs a sufficient overpotential and (2) holes will
lose part of their oxidative capacities during the transfer
Received: February 22, 2019
© XXXX American Chemical Society
A
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process in a composite semiconductor system.29 For instance,
Shaobin Wang et al. found that 3D hierarchical microsphere
rGO/Bi2MoO6 presented 18 μmol/L of O2 evolution after 3 h
of 300 W xenon lamp irradiation while there is a slight
decrease in activity after four-time recycling tests.30 Fur-
thermore, the O2 producing process is far slower than the
photogenerated carrier recombination. The oxygen production
is second-order, which is far below the submillisecond-order
rate of electron−hole combination.31,32 Therefore, the
presence of an electron transporter is critical to boost the
efficiencies for photocatalytic water oxidation by effective
electron relaying. The 2D planar π-conjugated structure of g-
C3N4 is proven to have good electronic conductivity.33,34 At
the same time, it has a high specific surface area and superior
adsorption capacity. Combining the unique graphene-like g-
C3N4 with Bi2MoO6 is a promising option because of the
better light harvesting and excellent photocatalytic activities.
One has witnessed many efforts in obtaining significant
degradation for various organic pollutions.35 g-C3N4/
Bi2MoO6 was reported to exhibit superior photocatalytic
activity toward the degradation of dyes (Rhodamine B and
methyl blue) under visible light irradiation. Zhou et al. used a
g-C3N4/Bi2MoO6 film as photocatalyst that degraded methyl-
ene blue (MB) in aqueous solution under visible light
irradiation with a photocatalytic ability superior to the single-
component photocatalyst.36 Hou et al. used g-C3N4/Bi2MoO6
to decompose Rhodamine B with visible light irradiation,
showing a photocatalytic activity higher than that of pure g-
C3N4 or Bi2MoO6.37 Nonetheless, g-C3N4/Bi2MoO6 is barely
considered to be highly active for water oxidation because g-
C3N4 has a low O2 evolution ability due to its self-
decomposition by photoholes although it has been considered
as a highly efficient photocatalyst for H2 generation.38−40 It
should be noted that Ru complexes are distinguished catalysts
in the aspect of water oxidation, despite their unstability under
existing catalytic devices.41 Moreover, their stability can be
improved by combining them with semiconductors. Numerous
studies about the Ru complexes as the molecular photo-
sensitizers for electron-transfer processes have been reported in
the past few years in the field of photocatalysis, for instance,
[Ru(bpy)3]2+/3+-type complexes.12−14 Inspired by the Ru
complex for water oxidation, loading metallic Ru or its oxide
RuO2 on g-C3N4 sheets is expected to be feasible to improve
the stability of materials and give an easy transmission for
photoexcited electrons. Meanwhile, the metallic Ru or RuO2
can be used as the active sites of water oxidation.40
2. EXPERIMENTAL SECTION
All the chemicals are analytically pure and were used directly after
purchase.
2.1. Synthesis. The pure g-C3N4 was obtained through heating
melamine with a speed of 5 K·min−1 up to 823 K for 4 h. The
melamine was hydrothermally treated for 24 h at 453 K in advance by
deionized water.
The single Bi2MoO6 was synthesized by hydrothermal self-
assembly method. First, Bi(NO3)3·5H2O (A) and NaMoO4·2H2O
(B) with a mole ratio of 2:1 were dissolved into an equal volume of
ethylene glycol solution, respectively. Then the A solution was added
into the B solution drop by drop. After the mixture solution was
stirred for 30 min, an equal volume of ethanol (99.7 wt %, analytical
grade, Tianjin Fengchuan Chemical Reagent Technologies Co. Ltd.)
was added. Last, the above mixture solution was transferred into a
high pressure reactor with Teflon as liner (HPRT) and heated for 12
h at 433 K. The pale yellow power material was obtained through
washing three times with deionized water.
The binary Bi2MoO6/g-C3N4 was prepared by heating the different
mass ratio of Bi2MoO6 and g-C3N4 at 433 K for 12 h with deionized
water as solvent in HPRT. The pale yellow power Bi2MoO6/g-C3N4
was dried in an oven at 333 K for 12 h. A series of Bi2MoO6/g-C3N4
materials with different mass ratio (Bi2MoO6:g-C3N4 = 5/6, 5/5, 5/4,
5/3, 5/2 and 5/1) were prepared and used as Bi2MoO6/g-C3N4-x,
where x is representing the mass ratio.
The Ru/Bi2MoO6 and Ru/g-C3N4 were obtained by reduction of
ruthenium(III) trichloride (RuCl3) with sodium borohydride
(NaBH4) in solution. A RuCl3 solution (5.00 mmol·L−1) with a
different volume was added to the Bi2MoO6 aqueous solution to form
each Ru loading material. Then 0.01 mol·L−1 NaBH4 solution was
dropwise into the above solution until RuCl3 was reduced to Ru0
under continuous stirring 5 h. The products were washed three times
with distilled water and dried in an oven overnight (12 h) at 333 K to
obtain Ru/Bi2MoO6 with different Ru loading contents (that is, 0.38,
1.14, 1.90, and 2.66 wt % according to the dosing ratio), used as Ru/
Bi2MoO6-x, where x represents the dosing mass ratio. For
comparison, a Ru/g-C3N4 sample with a mass ratio of 1.9 wt %
(Ru to g-C3N4) was prepared by a similar procedure, used as Ru/g-
C3N4 1.9 wt %.
The Bi2MoO6/Ru/g-C3N4 was obtained by heating the same mass
ratio of Ru/Bi2MoO6 and g-C3N4 at 433 K for 12 h with deionized
water as solvent in HPRT. Then, the product was naturally cooled to
the environment temperature and washed three times with deionized
water. Finally, pale yellow power Bi2MoO6/Ru/g-C3N4 was obtained
after drying in an oven overnight (12 h) at 333 K.
The used characterization methods are provided in the Supporting
Information (Experimental Section).
2.2. Photocatalytic Test. The photocatalytic activity was assessed
by photocatalytic water oxidation using a vacuum closed circulation
system as shown in Figure S1. The catalysts (0.05 g) were evenly
dispersed in 100 mL of solution, and the pH was adjusted to be 10
with the help of 5 mol·L−1 NaOH solution. NaIO3 (0.396 g) was used
as an electron trapping agent. The mixing solution was deoxidation by
bubbling with N2 gas about 30 min in a glass reactor (200 mL). The
quartz sheet served as light transmission window and the seal cover.
The reaction was processed under visible light irradiation (wavelength
>420 nm, 300 W xenon light source, beam diameter 5 cm, 30 mW·
cm−2) at room temperature. Then the xenon light source was set at a
10 cm distance from the liquid level of the solution. The productivity
of O2 was repeatedly measured at an interval of 1 h with the online
gas chromatography connected to the closed reactor within
continuous 7 h visible light irradiation. The generated O2 was
separated by passing through a 2 m × 3 mm packed 5A column of
molecular sieves with Ar carrier gas (high purity 99.999%) and
measured by gas chromatography with a thermal conductivity
detector (TCD).
In this work, a new efficient Z-scheme water oxidation
photocatalyst Bi2MoO6/Ru/g-C3N4 was constructed. The
excellent O2 evolution was achieved by linking the highly
oxidative catalyst Bi2MoO6 and the high electron transfer g-
C3N4 with the metallic Ru’s aid. The prepared catalyst had
been used to explore the photocatalytic performance of water
for oxygen production under visible light irradiation. A well-
designed synergistic mechanism was proposed, including the
Schottky barrier built in the interface between Ru and
semiconductor and the unobstructed electron transfer channel
formed across the interfacial domain, which could promote
efficiently O2 generated activity by well separating the
photogenerated carriers and the very positive overpotential
in the oxidative terminal.
2.3. Computational Details. The electronic structure, energy
band structure, density of state, and work functions of Ru (100),
Bi2MoO6 (100), Ru/Bi2MoO6, and Ru/g-C3N4 were calculated by
B
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Scheme 1. Synthetic Process of Ternary Catalyst Bi2MoO6/Ru/g-C3N4
Figure 1. (a) XRD patterns of g-C3N4, Bi2MoO6, Bi2MoO6/g-C3N4 5/5, Ru/g-C3N4 1.90 wt %, Ru/Bi2MoO6 1.90 wt %, and Bi2MoO6/Ru/g-
C3N4. The vertical lines color of red and black represented the stand cards of Ru (JCPDS 03-065-7646) and Bi2MoO6 (JCPDS 01-072-1524). (b)
XPS spectrum of Ru and C in Bi2MoO6/Ru/g-C3N4.
density functional theory (DFT), which was performed with the
CASTEP code of Materials Studio. The 3 × 1 × 1 supercells for the
pure phase of Ru and Bi2MoO6 have been built. A perpendicular
vacuum between neighboring slab images was 20 Å for the slab model.
For the models of Ru/Bi2MoO6 and Ru/g-C3N4, the six-atom Ru
cluster was established with a simple shape and the lengths of the x, y,
and z axes are all 4 Å. Then the Ru cluster was placed on the surface
of Bi2MoO6 (or g-C3N4) supercell, and the charge density difference,
band structure, and density of state for g-C3N4, Ru/g-C3N4, Bi2MoO6,
and Ru/Bi2MoO6 were calculated through the energy task. The
Perdew−Burke−Ernzerhof (PBE) parametrization of the generalized
gradient approximation (GGA) is used as the optimization functional.
The electron wave function is expanded in plane waves up to a cutoff
energy of 420 eV in the case of ultrafine potential. Calculations of the
lattice parameters, as well as the atomic position relaxations, were
carried out until the forces and total energy converged within 0.015
eV/Å and 10−5 eV, respectively. The work function was calculated by
population analysis and obtained through the CASTEP Analysis-
Potentials module.
Bi2MoO6. For the binary (Bi2MoO6/g-C3N4 5/5, Ru/Bi2MoO6
1.90 wt %, Ru/g-C3N4 1.90 wt %) and ternary composite
(Bi2MoO6/Ru/g-C3N4), no redundant peaks are observed,
except for the separated components Bi2MoO6, g-C3N4, and
Ru. In addition, for g-C3N4 supported samples, the expanded
peak at 27.4° vests in the (0 0 2) plane and ascribes to
superposition of conjugated aromatic systems (Figure S2). For
the Ru-loaded samples, Ru/g-C3N4 1.90 wt % (Figure 1a), Ru/
Bi2MoO6 (Figure S3), and Bi2MoO6/Ru/g-C3N4 (Figure 1a),
the characteristic peak at 44.02° of Ru (JCPDS 03-065-7646)
could be found, indicated that Ru being successfully
compounded together with Bi2MoO6 and g-C3N4.
The X-ray photoelectron spectroscopy (XPS) was further
measured to explore the elements valence state of Bi2MoO6/
Ru/g-C3N4 (Figure 1b and Figures S4 and S5). The survey
spectrum shows all elements in the Bi2MoO6/Ru/g-C3N4
(Figure S4). The signals of 158.91 and 164.22 eV vest in the
Bi 4f7/2 and Bi 4f5/2 of the Bi3+ 42 while the peaks at 232.20 and
235.35 eV are attributed to the Mo 3d5/2 and Mo 3d3/2 of the
sample.29 The signals at 530.04, 531.31, and 532.28 eV vest in
the lattice oxygen, hydroxyl oxygen, and adsorbed oxygen of
the Bi2MoO6 surface (Figure S5, parts a, b, and c).43 The peak
of N is fitted into three peaks (Figure S5d) where the binding
energies at 398.46 eV vest in the sp2 of (CN−C)38 and the
signals of 398.89 and 400.58 eV vest in g-C3N4 just as in the
recited analysis.39 As shown in Figure 1b, the signals at 284.66
and 288.21 eV belong to the C 1s and the sp2 hybridization of
(C-(N)3) for g-C3N4. At the same time, the weak peaks at
279.97 and 284.07 eV are assigned to Ru 3d5/2 and Ru 3d3/2 of
the element Ru0.44 The information from XPS and XRD is the
3. RESULTS AND DISCUSSION
3.1. Morphologies and Structure of the Prepared
Materials. The hollow spherical Bi2MoO6 was prepared by
the hydrothermal self-assembly method. Then the catalyst
Bi2MoO6/Ru/g-C3N4 was obtained by gentle hydrothermal
synthesis method with g-C3N4 and Ru/Bi2MoO6 prepared in
advance (Scheme 1). The prepared materials were explored by
X-ray diffraction (XRD) in Figure 1a. There are two peaks at
2θ = 13° and 27.5° of g-C3N4 (JCPDS 87-1526),
corresponding to the (1 0 0) and (0 0 2) planes of g-C3N4,
respectively. For the pure sample Bi2MoO6, all the signals
correspond to the standard card JCPDS 01-072-1524 of
C
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strong evidence that the metallic Ru particles were successfully
loaded to Bi2MoO6/g-C3N4 and formed the trinary catalyst
Bi2MoO6/Ru/g-C3N4 according to the present preparation
method.
Furthermore, the light absorptive performances of the
prepared catalysts were measured through UV−vis diffuse
reflectance spectroscopy (Figure 2 and Figures S6 and S7). It
microspheres, which are assembled by a large set of
nanoplatelets (Figure 3b). When Bi2MoO6 is covered with g-
C3N4 (Figure 3c), the assembled materials mostly maintain the
original appearance of the separated components with a close
contact with each other whereas part of Bi2MoO6 decomposes
into small pieces and distributes over the g-C3N4 surface.
Interestingly for Bi2MoO6/Ru/g-C3N4, Ru was well attached
to Bi2MoO6 and partially links with g-C3N4, which was marked
with the magenta virtual coil in Figure 3d, confirming that
most of the Ru particles preferentially adhered to Bi2MoO6.
Similar results can be observed in the samples TEM and
HRTEM (Figure 4). The ternary catalyst is distributed
Figure 2. UV−vis diffuse reflectance spectroscopy of the prepared
materials.
Figure 4. TEM images (a) and HRTEM images (b) of Bi2MoO6/Ru/
g-C3N4.
was observed that the absorption edges of g-C3N4 and
Bi2MoO6 were 485 and 467 nm, respectively. For Ru-loaded
catalysts, the light absorption intensity of the catalysts
increased with the increase of Ru content at 500 to 800 nm
(Figure 2 and Figure S7), which indicated that the Ru-loaded
samples have a good light response. Moreover, the ternary
composite has the strongest absorption at the visible region
than other samples. It indicated that the Bi2MoO6/Ru/g-C3N4
could well harvest visible light to produce photogenerated
electrons and holes for the catalytic reaction.
The morphologies of the as prepared catalysts were explored
through the scanning electron microscope (SEM) and
transmission electron microscopy (TEM). Figure 3a shows
that g-C3N4 presents a typical irregular agglomerated piece
structure. And Bi2MoO6 is in the form of petal-shaped hollow
uniformly in the field of view (Figure 4a), the small pieces
of Bi2MoO6 exhibit microspheres and are scattered in
crumbled multilayer sheets of g-C3N4 film. It is observed
that the diameter of Ru particles is about 10 nm from the
HRTEM (Figure 4b). The stripe width of 0.315 nm is
attributed to the (1 3 1) plane of Bi2MoO6, and 0.234 nm is
assigned to the (1 0 0) plane of Ru element (JCPDS 03-065-
7646) in the HRTEM images of Figure 4b. Thus, it is indicated
the three-phase materials are in uniform contact, and Ru
particles as a linkage bridge may play a key role for efficient
transfer of the interface electrons.
For further investigation of the elemental distribution in the
ternary material, STEM-EDX chemical mapping was studied.
Figure 5a is the area photograph of Bi2MoO6/Ru/g-C3N4,
including as much as possible the three kinds of components.
The weight percentages of all elements in Bi2MoO6/Ru/g-
C3N4 are 22.65, 35.25, 1.97, 27.45, 6.35, and 6.33% for C, N,
Ru, Bi, Mo, and O (Figure 5, parts b and c), respectively,
basically consistent with the theoretical value. In addition, the
amount of Ru loading in Bi2MoO6/Ru/g-C3N4 sample was
measured to be 1.46 wt % with inductively coupled plasma
optical emission spectrum (ICP-OES), presenting consistency
with both EDX and the initial dosing quantity. The elemental
distributions of C and N have close consistent coordinates at
the same area of the map, assigned to a collection of g-C3N4
(Figure 5, parts d and e). The elemental distribution profiles of
Bi, Mo, and O present rough concordant patterns, belonging to
a assembly of Bi2MoO6 (Figure 5, parts f−h). Meanwhile, it
was observed that most of Ru particles distributed in nearly
same locations with Bi2MoO6 (Figure 5i), inferring the strong
interactive existence of Ru particles with Bi2MoO6 to inhibit
Ru’s migration, consistent with the result of SEM (Figure 3d)
and TEM (Figure 4d).
Figure 3. SEM images of g-C3N4 (a), Bi2MoO6 (b), Bi2MoO6/g-C3N4
5/5 (c), and Bi2MoO6/Ru/g-C3N4 (d). The magenta marked area
represents Ru particles.
3.2. Activity Assessing of the Photocatalytic Reac-
tion. Photocatalytic water oxidation tests were carried out
D
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Figure 5. STEM-EDX chemical mapping of Bi2MoO6/Ru/g-C3N4 (a, b), the weight percentages of all elements in Bi2MoO6/Ru/g-C3N4 (c), and
the separated elemental distributions of C, N, Bi, Mo, O, and Ru (d−i).
Figure 6. (a) Oxygen productivity of the as prepared catalysts with visible light irradiation. (b) Cycling experiment of Bi2MoO6/Ru/g-C3N4.
using NaIO3 as a sacrificial electron acceptor at room
temperature to investigate the activities of the prepared
materials. The catalysts were evenly dispersed in 100 mL
solution under visible light irradiation. The productivity of O2
was measured by online gas chromatography connected to the
closed reactor (Figure S1). The maximum O2 evolution
amounts are 104.55 (Bi2MoO6), 12.91 (g-C3N4), 39.92
(Bi2MoO6/g-C3N4 5/5), 119.11 (Ru/Bi2MoO6 1.90 wt %),
20.19 (Ru/g-C3N4 1.90 wt %) and 328.34 μmol·g−1·h−1
(Bi2MoO6/Ru/g-C3N4) after visible light irradiation, respec-
tively (Figure 6a). Obviously, the O2 production is poor for
either the sole Bi2MoO6 or g-C3N4 and even the composites
Bi2MoO6/g-C3N4 (Figure S8). However, oxygen evolution is
slightly promoted by loading metallic Ru on both Bi2MoO6
and g-C3N4 (Figures S9 and S10). More importantly, after
loading metallic Ru to the binary composite, the oxygen
production of Bi2MoO6/Ru/g-C3N4 could be quickly
promoted (Figure 6a), 25.33 times as much as g-C3N4 and
3.14 times as much as single Bi2MoO6. Obviously, the excellent
oxygen production efficiency of Bi2MoO6/Ru/g-C3N4 gives
credit to the Ru-assisted electron transport and the composite
effect of the separated component. It is inferred that the
ternary Bi2MoO6/Ru/g-C3N4 can accelerate the separation of
carriers in such Z-scheme structure, leading to the increased
catalytic efficiency. It is well-known that water oxidation
proceeds according to the following reaction formula: 2H2O +
4h+ → O2 + 4H+.45,46 Therefore, it is speculated that by
consuming excess H+, it will speed up the photocatalytic water
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Figure 7. Photoluminescence spectra (a), the time-resolved fluorescence decay spectra (b), the transient photocurrent recorded (c), and the EIS
Nyquist plots (d) of the as-prepared materials.
oxidation reaction rate. Therefore, the oxygen production of
Bi2MoO6/Ru/g-C3N4 was investigated under different pH
values (Figure S11). The oxygen production of the catalyst
gradually increases with the increase of the pH value, and it
reaches a maximum when the pH value is 10. It indicates that
the alkaline environment helps to accelerate the rate of the
oxygen generation by photocatalytic water oxidation. To
explore the stability of Bi2MoO6/Ru/g-C3N4, the reaction
was performed over seven recycling tests. As illustrated in
Figure 6b, more than 91.4% of the photocatalytic activity of
Bi2MoO6/Ru/g-C3N4 after seven runs could still be retained.
The initial and used Bi2MoO6/Ru/g-C3N4 catalysts were
examined by XRD (Figure S12) and SEM (Figure S13). No
significant change in either crystal phase structure or the
morphology of the catalysts is observed, confirming that the
photocatalyst, Bi2MoO6/Ru/g-C3N4, is quite stable under
reactive conditions. Furthermore, the O2 evolution activities
from the references for several semiconductor catalysts and Ru-
based composite catalysts were listed in the Supporting
Information, Table S1, for comparative purposes. Obviously,
our sample Bi2MoO6/Ru/g-C3N4 presents an attractive
performance as a promising water oxidative photocatalyst.
3.3. Investigation of the Photocatalytic Water
Oxidation Mechanism. Photoluminescence spectra (PL)
and the time-resolved fluorescence decay spectra were tested
to characterize the recombination and transfer behaviors of
photogenerated carriers of the prepared Bi2MoO6/Ru/g-C3N4
(Figure 7, parts a and b). The PL properties for Bi2MoO6/g-
C3N4 and Ru/Bi2MoO6 with the different composition ratio
were also investigated and displayed in Figures S14 and S15,
respectively. The fluorescence intensity of Bi2MoO6/Ru/g-
C3N4 is greatly reduced with a longest lifetime of fluorescence.
It infers that the carriers’ recombination rate of ternary
material is absolutely the lowest among these prepared
samples. Furthermore, from transient photocurrent testing
(Figure 7c), we have also observed the ternary material
exhibits the highest transient photocurrent than any single and
binary samples and has a tendency toward stabilization,
evidencing there is a synergistic effect among the constitutions
to inhibit the recombination of photoexcited carriers. It is also
noted that g-C3N4 presents the lowest photocurrent density,
while via loading Ru, the separation efficiency of the
photogenerated carriers does really achieve high promotion.
Furthermore, electrochemical impedance spectroscopy (EIS) is
used to explore our samples conductive properties. Figure 7d
shows the EIS Nyquist plots of as prepared materials under
illumination. The arc radii of the samples loaded with Ru are
quit smaller than those without Ru loadings. It indicates that
the introduction of Ru helps to decrease resistance of the
electrons migration, and greatly promote the separation
efficiency of carriers. Meanwhile, it is more notable that
Bi2MoO6/Ru/g-C3N4 has the smallest arc radius in all samples,
implying the lowest interface resistance and the fastest
interfacial electronic transfer. From the photoelectrochemical
testing, one can deduce that the Ru loading and constituent
synergy in Bi2MoO6/Ru/g-C3N4 composite play the crucial
roles to its efficient photoelectric conversion property and
excellent electron transport capability.
To investigate sample redox, the polarization curves of the
materials were tested in the dark (Figure S16). As we know, a
positive current less than 1.4 V vs Ag/AgCl can favor oxygen
evolution.31,32 The catalytic water oxidation voltage of
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Figure 8. Charge density difference of Ru/g-C3N4 (a) and Ru/Bi2MoO6 (b) and the band structures of g-C3N4 and Ru/g-C3N4 (c) and Bi2MoO6
and Ru/Bi2MoO6 (d).
Figure 9. Plots of transformed Kubelka−Munk function vs the energy for g-C3N4 and Bi2MoO6 (a), VB-XPS spectra of the catalysts g-C3N4 and
Bi2MoO6 (b), and the electrostatic potential of Bi2MoO6 (100) face (c) and Ru (100) face (d).
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Figure 10. •O2− (a) and •OH (b) radical active species tested by ESR with the visible light irradiating for 10 min over Bi2MoO6/g-C3N4 5/5 and
Bi2MoO6/Ru/g-C3N4.
Bi2MoO6/Ru/g-C3N4 is tested at onset potential (current
value: zero) to be about 0.9 V. Moreover, the current density is
1.0 mA·cm−2 for Bi2MoO6/Ru/g-C3N4, whereas in contrast,
only a very minimum current (about 0.04 mA·cm−2) is
obtained for Bi2MoO6/g-C3N4. Obviously, the current density
is stronger and the overpotential is more negative for Ru
loading samples than those without Ru loading. It clearly
indicates that Ru component in the composites can be easier to
promote multiple-electron water oxidative ability via an
excessive driving potential.
electron transfer are shown in Figure S18. When we combine
Bi2MoO6 and g-C3N4 to build a p−n junction, electrons will
flow from g-C3N4 to Bi2MoO6 (Figure S18a). As a result, the
bands are totally moved up for Bi2MoO6 and moved down for
g-C3N4 until the thermodynamic equilibrium being reached.
The valence band potential of g-C3N4 is lower than the
oxidation−reduction potential of •OH/OH− (2.40 eV vs
NHE) and the conduction band potential of Bi2MoO6 is more
positive than O2/•O2 (−0.33 eV vs NHE).47,48 Thus, this
−
binary composite is expected to produce less O2, conformed to
the experimental results (Figure 6a), which could be convinced
Furthermore, to theoretically investigate the recombination
of photocatalytic e-h pair, the charge density difference, band
structure, and density of state for g-C3N4, Ru/g-C3N4,
Bi2MoO6, and Ru/Bi2MoO6 were calculated by DFT with
the CASTEP code of Materials Studio. For both Ru/g-C3N4
and Ru/Bi2MoO6 (Figure 8a, b), obviously, the electronic
densities of Ru atoms are decreased while those in substrates g-
C3N4 and Bi2MoO6 are increased. It indicates that the
electrons of Ru atoms are easily transferred to substrate (g-
C3N4 or Bi2MoO6). Interestingly, for Ru/Bi2MoO6, the Ru
atoms form a strong chemical bond with the O atom from
substrate Bi2MoO6 whereas the case is obviously different for
Ru/g-C3N4, confirming Ru preferentially adheres to the
Bi2MoO6 surface with a strong interaction which is consistent
with the results of SEM (Figure 3d). Furthermore, the energy
band structures (EBS) and density of state (DOS) of g-C3N4,
Ru/g-C3N4, Bi2MoO6, and Ru/Bi2MoO6 are obtained (Figure
8, parts c and d, and Figure S17). By analyzing EBS and DOS,
it is obvious that with Ru addition, the band gaps of Ru/g-
C3N4 and Ru/Bi2MoO6 decrease, supporting the results of a
wide solar harvesting performance from UV−vis diffuse
reflectance spectroscopy. In addition, the energy states in VB
for Ru/g-C3N4 and Ru/Bi2MoO6 are all significantly dense,
which are expected to increase the mobility of photogenerated
electrons and thus effectively reduce the recombination of e-h
pairs.
•
−
•
by both weak signals of the O2 and OH species under
visible light irradiation in ESR test (Figure S19, parts a and b).
Furthermore, something completely different happens if we
assemble Bi2MoO6 and g-C3N4 through a Ru bridge. In order
to explore the band energy aligning in such complex system,
the work functions of Bi2MoO6(100) and Ru(100) are
calculated by the CASTEP Analysis-Potentials module and
they are 6.160 and 5.203 eV, respectively (Figure 9, parts c and
d). The work function of g-C3N4 was 4.025 eV as our previous
report.35 As we all known, the greater the work function of the
materials is, the stronger the binding electron capacity is, and
the more difficult it is for the electron to get away from the
material. Therefore, in terms of the assessment of the work
function, it is concluded that Bi2MoO6 has the stronger
binding electron ability. Meanwhile, the VB of Bi2MoO6 is
involved in the existing elements including O, Bi, and Mo as
observed in Figure S20. Furthermore, the Fermi energy (Ef)
could be calculated from these work function values according
to the following formula:
Ef = Evacuum − Φ
Here Evacuum and Φ represent the energy of a vacuum
stationary electron and work function, respectively. The Ef of
Bi2MoO6, Ru, and g-C3N4 were obtained as 1.66, 0.703, and
0.475 eV vs NHE, respectively (Figure 9, parts c and d).
Obviously, the Fermi level of Ru is higher than a p-type
Bi2MoO6 and lower than a n-type g-C3N4 (Figure S18b). So
the band potentials of Bi2MoO6 are bent downward when
contacted with Ru, and the Schottky barrier is established
between Ru and p-Bi2MoO6, which holds back photoexcited
holes, but electrons can flow from Bi2MoO6 to Ru. Meanwhile,
the band potentials of g-C3N4 are bending upward, and thus
the electrons move into Ru is not allowed owning to the
existence of the Schottky barrier. Conclusively, the electron
could easily transfer from Bi2MoO6 via Ru to g-C3N4 which is
For further determining the transfer path of carriers, the
band potentials of the single catalysts were measured by UV−
vis diffuse reflectance spectroscopy and the VB-XPS spectrum
(Figure 9). The band gaps (Eg) of g-C3N4 and Bi2MoO6 are
2.47 and 2.65 eV, respectively (Figure 9a). The valence band
(VB) values of g-C3N4 and Bi2MoO6 are tested to be 1.51 and
2.43 eV (versus NHE) while the conduction band (CB) values
of g-C3N4 and Bi2MoO6 are counted as −0.96 and −0.22 eV,
respectively (Figure 9b). According to the alignment of their
band levels, the illustrated pathways about the interface
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Article
Figure 11. Structure of the Z-scheme catalysts and the oxygen production mechanism for Bi2MoO6/Ru/g-C3N4.
boosted by both inner interface electric field and the Schottky
barrier. Obviously, the CB electrons of g-C3N4 and the VB
holes of Bi2MoO6 are not transported, and thus the strong
redox properties are retained simultaneously. It is evidenced by
that at the same reaction conditions (Figure S21), only without
NaIO3, 1 μmol of O2 product was obtained after 1 h visible
light irradiation, which supported a rapidly transferred route
through the Z-scheme (Bi2MoO6 → Ru → g-C3N4) due to the
good conductive behavior of g-C3N4. It is indisputable that the
recombination of photocarriers is effectively inhibited and the
multiple-electron water oxidative proceeding O2 is largely
promoted due to the preferential transfer and controllable
retention of photoexcited carriers in each redox terminals.
−
the strong signals of DMPO−•O2 and DMPO−•OH of
ternary catalyst Bi2MoO6/Ru/g-C3N4 under the 10 min visible
light irradiation from EPR testing (Figure 10 and Figure
S19c,d). In this case, a Ru sandwiched p−n junction was built
in the ternary catalyst, and the metallic Ru could provide a
powerful bridge for efficient transfer of the interface electrons.
Therefore, the catalyst Bi2MoO6/Ru/g-C3N4 benefits, to lower
the recombination rate of carriers as well as maintain a high
redox capability.
4. CONCLUSIONS
In summary, we have successfully constructed a ternary
Bi2MoO6/Ru/g-C3N4 with excellent activity for O2 evolution
through water splitting under visible light irradiation via
assembling metallic Ru into the p−n junction of Bi2MoO6/g-
C3N4. As is expected, Bi2MoO6/Ru/g-C3N4 shows a durable
excellent O2 productivity with a value of 2298.38 μmol·g−1
after 7 h of visible light irradiation and seven repeated uses,
absolutely higher than that of the separated single components
and binary composites. In such a ternary system, the loading
Ru particles could form a Schottky barrier with both p-type
Bi2MoO6 and n-type g-C3N4, which has been aligned with a p−
n junction due to the matched band levels. United with a good
conductive behavior of g-C3N4, a rapid transfer route of the
photoelectrons through Bi2MoO6 → Ru → g-C3N4 → NaIO3
is formed. In such a particular structure, the preferential
transfer and controllable retention of photoexcited carriers in
each redox terminals is well achieved. Thus, the electron−hole
pairs are efficiently separated without the loss of the high redox
capacity, as well as largely promoting the multiple-electron
water oxidative reaction. It is proposed that the Ru loading and
constituent synergy in Bi2MoO6/Ru/g-C3N4 composite play
the crucial roles to its enhanced visible light utilization,
efficient photoelectric conversion, excellent photogenerated
carrier separation, and electron transport capability.
On the basis of the above discussion, the photocatalytic
water oxidation mechanism is depicted in Figure 11. With
visible light irradiating, either Bi2MoO6 or g-C3N4 can be
readily excited to form carries own to its narrow band gap. The
VB value of Bi2MoO6 is effective for the O2 generation due to
the more positive potential than that of H2O/O2 (1.23 eV vs
NHE),42 as shown by a better O2 yield of 104.55 μmol·g−1·h−1
(Figure 6). However, the recycle activity is quickly deceased
due to the slower oxidative proceeding compared with the
quick recombination. For a sole g-C3N4, it has poor water
oxidation ability as indicated by weak O2 production (12.91
μmol·g−1·h−1), which is the result of the lower positive
potential compared with that of H2O/O2. For Bi2MoO6/g-
C3N4 (Figure 11a), the electrons could be collected on CB of
Bi2MoO6; however, the holes will move to VB of g-C3N4 via a
p−n junction contact, which greatly enhanced the electron and
hole separation efficiencies. Unfortunately, as the electrons/
holes move toward the more positive/negative energy bands,
the electronic reduction and the hole oxidizing ability are
greatly weakened. Therefore, a bad O2 evolution (39.92 μmol·
g−1·h−1) of Bi2MoO6/g-C3N4 is observed. Naturally, the O2
evolution is not promoted by Ru loading in sole Bi2MoO6 due
to the fact that recombination could not be inhibited.
Meanwhile Ru/g-C3N4 is also unable to split water to produce
O2 because the valence potential (h+) of g-C3N4 could not be
shifted to positive and to beneficial water oxidation via Ru
loading although the carriers’ separation can indeed be
promoted. As is very interesting for ternary catalyst (Figure
11b), the Ef of Ru (0.475 eV) is between the CB of p-type
Bi2MoO6 and VB of n-type g-C3N4. Thence, this helps to
quickly make the photogenerated electrons from CB of
Bi2MoO6 flow into metallic Ru through a Schottky barrier49
and then combine with photogenerated holes from g-C3N4,
leaving the holes in VB of Bi2MoO6 with stronger oxidative
ability for further water oxidation reaction. It also was noticed
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00524.
Details of catalyst characterization, experiment device
diagram, XRD patterns, XPS survey spectrum and the
enlarger images of the separated elements, UV−vis
diffuse reflectance spectra, time courses of O2 evolution,
SEM images, photoluminescence spectra, linear sweep
voltammetry (LSV) curves, the total density of state
I
DOI: 10.1021/acs.inorgchem.9b00524
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Inorganic Chemistry
(TDOS) and projected density of state (PDOS),
Article
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with the values reported in the literature (PDF)
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tailor-made molecular ruthenium catalyst for the oxidation of water
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AUTHOR INFORMATION
■
(14) Thompson, D. W.; Ito, A.; Meyer, T. J. [Ru(bpy)3]2+ and other
remarkable metal-to-ligand charge transfer (MLCT) excited states.
Pure Appl. Chem. 2013, 85, 1257−1305.
Corresponding Author
*(X.W.) E-mail: wang_xiao_jing@hotmail.com. Fax: +86-471-
4994406. Telephone:+86-13948315232.
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ORCID
Yuhang Wu: 0000-0002-4339-0753
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is financially supported by the National Natural
Science Foundation of China (Grant Numbers 21777078 and
21567017) and the Project of Research and Development of
the Applied Technology for Inner Mongolia (201702112).
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into H2 and O2 under visible light over photocatalyst panels consisting
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